Optical element for a LCD display

ABSTRACT

An optical element for a display unit comprising a first rigid, transparent plate ( 9   b ) and at least one flexible optical foil ( 5 ), wherein the foil ( 5 ) is held to the plate ( 9   b ) by means of an underpressure created, for example, by a suction device ( 17 ). 
     The optical element according to the invention overcomes the problem of wrinkling foils by creating an underpressure that presses the foil ( 5 ) against the rigid plate ( 9   b ), thereby preventing the foil from wrinkling. 
     According to an embodiment of the invention, the optical element further comprises a second rigid, transparent plate ( 9   a ), said optical foil is arranged between the first and second plates, and the plates ( 9   a,    9   b ) are held together by means of underpressure.

The present invention relates to an optical element comprising a firstrigid, transparent plate and at least one flexible optical foil. Such anelement can be used in general for distributing light from a lightsource, and in particular in front of the back lighting device of anLCD.

The invention also relates to a method of manufacturing such an opticalelement and to a display unit, such as an LCD, with such an element.

Certain types of display units, in particular LCD units, require asource of back lighting to light up the screen. This back light isgenerated by a back lighting device arranged behind the screen. In thesecases, there is a need for directing the back light onto the screen, tominimize diffraction and reflection, and for this purpose, it is knownto arrange at least one optical foil in front of the back lightingdevice to influence the light path. Such foils are often made of a thinplastic sheet, with patterns of perpendicular lines formed thereon foraffecting any light passing through the foil.

However, the foils tend to wrinkle under the influence of heat from theback lighting device, especially if the area of the foil is large. As aresult, the foils may distort the back light after some time ofoperation if the foils are heated too much.

The same problem is present, for example, in the art of so-calleddaylight devices. Such devices are designed similarly to the back lightdevice of an LCD and are arranged in the sealing of a room to createpleasant light conditions in doors. Just like the LCD of a monitor, suchlighting devices have a large active surface.

In order to alleviate the above problem of wrinkling foils, it is knownto place the foils between two glass plates, thereby forming an opticalelement, also known as an optical stack. Nevertheless, the problem ofwrinkling has not been satisfactorily overcome. The reason is that theglass plates cannot be made too thick because of requirements on weight,cost, and optical properties such as reflection and light absorption (anormal thickness of such plates is 1 mm). It is therefore difficult forthe plates to impose a strong enough force on the foils, especially ifthe area of optical element is large.

Attempts have also been made to glue a foil to a glass plate. However,surfaces of the foil formed with the optical patterns mentioned abovemust not be glued, as the glue would distort the desired optical effect.As a consequence, this solution cannot be used in a case where thesurface of the foil facing the plate is patterned, nor where severalfoils are arranged in a stack.

It is an object of the present invention to alleviate the above problemand to provide an optical element that is less susceptible to wrinklingthan conventional elements.

According to a first aspect of the invention, this and other objects areachieved with a method of manufacturing optical elements comprising afirst rigid, transparent plate, and at least one flexible optical foil,said method comprising a removal of air from between said plate and saidfoil.

According to a second aspect of the invention, this and other objectsare achieved with an optical element of the kind mentioned by way ofintroduction, wherein the foil is held to the plate by means ofunderpressure.

A display unit, such as an LCD, may advantageously be provided with anoptical element according to the invention or be used with the methodaccording to the invention.

The optical element according to the invention overcomes the problem ofwrinkling foils by creating an underpressure that presses the foilagainst the rigid plate thereby preventing the foil from wrinkling.

By this technique, the plate can be pressed against the foil withsufficient force to avoid wrinkles in the foil.

According to one embodiment, the optical element comprises a suctiondevice connected to said plate and arranged to remove air from betweensaid plate and said foil. This makes it possible to maintain the desiredunderpressure, even if the connection between the plate and the foil isnot completely airtight.

In order to eliminate the need for a suction device or to improve theeffect of the suction device, a seal may be provided along the edges ofsaid plate, sealing the plate against the optical foil. Even arelatively simple sealing can drastically enhance the envisaged effect.

According to an embodiment of the invention, the optical element furthercomprises a second rigid, transparent plate, while said optical foil isarranged between said first and second plates, said plates being heldtogether by means of underpressure.

In this configuration, the optical foil is held between the two plates,which are pressed together by the underpressure. Several foils can bearranged between the plates. Again, the underpressure may be maintainedby a suction device.

As was mentioned above, a seal along the edges of the plates caneliminate the need for a suction device or enhance the effect of thesuction device. In the embodiment with two plates, the sealing ispreferably arranged between the two plates, thereby sealing off thespace which is formed between them and in which the foil(s) is (are)arranged.

The optical element may have any suitable number of foils withoutreducing the advantages of the concept. It may also comprise a so-calledhoneycomb, i.e. an element of, for example, aluminum with a plurality ofholes formed therein, for securing a straight path of light from theback lighting device. Such a combination of elements, sometimes referredto as an optical stack, can improve the qualities of a lighting devicein general and of a back lighting device of an LCD in particular. Byimplementing the present invention, these qualities are further improvedand made independent of heat variations.

In the embodiment with only one plate, the honeycomb, which by itsconstruction allows air to pass through easily, is preferably arrangedbetween the first plate and the foil. In the embodiment with two platesforming the end plates of a stack, the honeycomb element and the foilsmay be arranged in any order that is found advantageous.

These and other aspects of the invention will be apparent from thepreferred embodiments described in more detail below with reference tothe appended drawings.

FIG. 1 is a schematic view of an LCD.

FIG. 2 is an exploded view of a first embodiment of an optical elementaccording to the invention.

FIG. 3 is a side elevation of the optical element of FIG. 1.

FIG. 4 is an exploded view of a second embodiment of an optical elementaccording to the invention.

The following description is based on the implementation of the opticalelement according to the invention in an LCD. It is emphasized that thisis only one possible implementation and that the optical elementaccording to the invention may also be used in other applications wherelight from a source is distributed.

FIG. 1 shows some of the parts of an LCD display, namely a back lightingdevice 1, an LCD screen 2, and an optical element 3, arranged in betweenthe lighting device 1 and the screen 2. The size of the screen dependson the application, but as an example the optical stack may beapproximately 90 cm by 50 cm.

With reference now to FIG. 2, the optical element in FIG. 1 comprises astack with two optical foils 5 and one honeycomb element 7 fittedbetween two transparent plates 9 a, 9 b. At least one of the plates isrigid in order to give the element structural strength. The opticalfoils are preferably made of plastic, and the plates are preferably madeof glass. These are the conventional materials chosen, allowing for acost-efficient and durable product.

The optical foils 5 in FIG. 2 each have a pattern of parallel groovesformed in at least one surface. Their purpose is to eliminatereflections in the back light from the lighting device 1. By arrangingtwo foils in sequence, with their patterns aligned differently, forexample perpendicularly, an even better effect is obtained.

The honeycomb element 7 may be made of aluminum and has a plurality ofhexagon-shaped cells, forming a honeycomb pattern, as shown in theexpanded section A. The cells are intended to ensure that the light fromthe back lighting device is straight. To improve their opticalcharacteristics, the cells may be coated with an light-absorbingcoating. In the preferred embodiment, the honeycomb is approximately 6mm thick and the cells are 3 mm in diameter.

The glass plates 9 a, 9 b are relatively thin, for example approximately1 mm thick, in order not to absorb or diffract the backlight to much.The glass plates, which are slightly bigger than the foils and thehoneycomb element, are provided along their edges with a sealing 11 of,for example, metal. The sealing may be taped to the glass with adhesivetape. If required, the sealing is improved with a flexible puttymaterial such as rubber or epoxy or an elastomer. The sealing 11 isadapted to make contact with both glass plates 9 a, 9 b when the opticalelement is assembled, thereby creating an essentially sealed space 13 inbetween the plates.

In view of the tolerances of the different components, for example ofthe honeycomb element 7, it may be advantageous to provide for a certainair gap between the plates. In this case it is preferred to add aflexible layer to the sealing, allowing for a further compression of thestack. The flexible layer may be allowed to harden while the stack ispressed together mechanically or otherwise.

With reference to FIG. 3, which is only schematic, a tube 15 is fittedin the sealing, thereby providing a channel 16 into the space 13 betweenthe plates in which the foils 5 and honeycomb 7 are arranged. A suctiondevice 17 (shown in FIG. 2), in the illustrated example a small bloweror fan, is connected to the tube 15 by means of a plastic tube 18 or thelike. The suction device 17 is adapted to remove air from the space 13,thereby pressing the plates 9 a, 9 b together.

In the currently implemented application, the blower 17 can bring thepressure in between the plates down to around 120 Pa, which has shown tobe a suitable pressure. It has been found advantageous to prioritize theair flow capacity of the blower rather than the pressure itself. Thus,even though the metal seal 11 is not completely airtight, a small(low-power) blower can remove air at a sufficiently fast rate topreserve the desired underpressure.

In trial applications, a blower from Delta Electronics with the modelnumber BFB0412HHA was found to be satisfactory. In order to reduce theadditional noise generated by such a blower, it was placed in a housingand provided with a damper (not shown).

Note that it is not necessary to maintain the underpressure between theplates when the LCD is not in use, as the foils are then not subject toheat from the back lighting device. It is therefore preferred to connectthe blower to the normal power supply of the LCD, activating the blowerwhen the display is turned on.

As mentioned above, it is also possible to embody the invention withouta suction device if a very tight seal can be arranged. The air betweenthe plates then only needs to be removed once, creating a vacuum betweenthe plates which will be maintained by the seal.

According to a second preferred embodiment of the invention, shown inFIG. 4, the optical element only comprises one glass plate, 109,provided with a sealing 111. It further comprises a stack with twooptical foils 105 a, 105 b, the outer foil 105 a being slightly largerthan the inner foil 105 b, thereby making contact with the sealing 111and forming a sealed space 113 between the plate 109 and the outer foil105 a. Like the optical element of FIG. 2, the element in FIG. 4 mayagain comprise a honeycomb element 107, in this case placed between theinner foil 105 b and the glass plate 109. The honeycomb 107 is similarin size to the inner foil 105 b.

A suction device 117 is connected to the sealing 111 and arranged toremove air from the space 113, just as air is removed from between theplates 9 a, 9 b in FIG. 2. The resulting underpressure in space 113presses the outer foil 105 a towards the plate and sandwiches the innerfoil 105 b and the honey comb 107.

As was noted above, the suction device may be left out if an airtightseal can be arranged between the plate and the foil and all air isremoved from between the plate and the foil.

It should be noted that the optical stacks described above are onlyexamples and that many alternative configurations are possible, such asa stack with a different number of foils, without a honeycomb element,or comprising a different type of component in the underpressure space13, 113.

1. An optical stack for use with a liquid crystal display, comprising: afirst rigid transparent plate having a perimeter; a second rigidtransparent plate having a perimeter; a first optical foil configured tosubstantially eliminate reflections, in a first direction, that areassociated with light from a light source; a collimator positionedbetween the first rigid transparent plate and the second rigidtransparent plate; and a seal positioned along the perimeter of thefirst rigid transparent plate and the perimeter of the second rigidtransparent plate; wherein the seal is configured to permit maintenanceof an atmospheric pressure differential between an interior area of theoptical stack and an area outside the optical stack, the interior areabeing defined by the first rigid transparent plate, the second rigidtransparent plate, and the seal.
 2. The optical stack of claim 1,further comprising a second optical foil configured to substantiallyeliminate reflections, in a second direction, that are associated withthe light from the light source.
 3. The optical stack of claim 2,wherein at least one optical foil of the first optical foil and thesecond optical foil is positioned between the first rigid transparentplate and the second rigid transparent plate.
 4. The optical stack ofclaim 3, wherein the collimator comprises a honeycomb structure.
 5. Theoptical stack of claim 4, wherein the seal is configured to permitattachment of an atmospheric pressure control device.
 6. The opticalstack of claim 5, further comprising an atmospheric pressure controldevice operatively connected to the seal and configured to adjustatmospheric pressure of the interior area of the optical stack.
 7. Theoptical stack of claim 6, wherein the atmospheric pressure controldevice is a blower that is configured to remove an atmospheric gas fromthe interior area.
 8. The optical stack of claim 7, further comprising abacklight configured to transmit light through the collimator.
 9. Theoptical stack of claim 8, further comprising a liquid crystal displayconfigured to use the backlight to form a viewable image.
 10. An opticalstack for use with a liquid crystal display, comprising: a rigidtransparent plate having a perimeter; a first optical foil configured tosubstantially eliminate reflections, in a first direction, that areassociated with light from a light source and having a perimeter; acollimator positioned between the first rigid transparent plate and thefirst optical foil; and a seal positioned along the perimeter of therigid transparent plate and the perimeter of the first optical foil;wherein the seal is configured to permit maintenance of an atmosphericpressure differential between an interior area of the optical stack andan area outside the optical stack, the interior area being defined bythe first rigid transparent plate, the first optical foil, and the seal.11. The optical stack of claim 10, further comprising a second opticalfoil configured to substantially eliminate reflections, in a seconddirection, that are associated with the light from the light source. 12.The optical stack of claim 11, wherein the collimator comprises ahoneycomb structure.
 13. The optical stack of claim 12, wherein the sealis configured to permit attachment of an atmospheric pressure controldevice.
 14. The optical stack of claim 13, further comprising anatmospheric pressure control device operatively connected to the sealand configured to adjust atmospheric pressure of the interior area ofthe optical stack.
 15. The optical stack of claim 14, wherein theatmospheric pressure control device is a blower that is configured toremove an atmospheric gas from the interior area.
 16. The optical stackof claim 15, further comprising a backlight configured to transmit lightthrough the collimator.
 17. The optical stack of claim 16, furthercomprising a liquid crystal display configured to use the backlight toform a viewable image.
 18. An optical element, comprising: first meansfor substantially eliminating light reflections in a first direction;second means for substantially eliminating light reflections in a seconddirection that is different from the first direction; means forcollimating light; and means for operatively securing together the firstmeans for substantially eliminating light reflections, the second meansfor substantially eliminating light reflections, and the means forcollimating light, wherein the means for operatively securing togetherincludes at least one transparent rigid member and a seal that isconfigured to permit maintenance of an atmospheric pressure differentialbetween an internal area of the means for operatively securing and anarea outside the internal area.
 19. The optical element of claim 18,further comprising means for providing light configured to transmitlight through the means for collimating.
 20. The optical element ofclaim 19, further comprising a liquid crystal display configured to usethe means for providing light to form a viewable image.